The present application is directed to an improvement in noise performance of imager systems such as, but not limited to, active-matrix flat panel imagers (AMFPI) based on amorphous and/or polysilicon. The described approach achieves an improved imager system, which operates in a charge gain (delivery) mode, and avoids cross-talk and other noise.
In a conventional amorphous silicon (a-Si) imager, a signal charge is transferred from a photodiode or hold capacitor through a single transistor to a data bus, connected to a charge detector. By holding the data bus voltage constant, charge is transferred to a charge detector circuit. Recently, work has been undertaken related to hybrid imagers. This type of imager uses transistors having distinct design configurations, and which may be made from different materials. The use of multiple transistors allows pixel gain, which can reduce the cost of the system electronics. For example, in one embodiment the hybrid imager may employ, but is not limited to, amorphous Si (a-Si) and polycrystalline-silicon (p-Si) transistors. The hybrid design enables pixels of the imager to increase (i.e., add a gain to) the generated signal. One particular type of hybrid imaging system uses a voltage amplifier, such as a source follower within a pixel that converts a signal to a voltage on a data bus. This same configuration can also be used to covert a signal to a current on a data bus. In this configuration, the amplifier is called a cascode. The difference between these two methods is determined by the impedance of the data bus: high for the voltage follower, low for the cascode amplifier.
Both of these systems suffer from drawbacks. The cascode is quite non-linear, and to linearize the system requires additional signal processing or the integration of additional electronic components within the pixel or detector circuit. The source follower has been used more successfully. It transfers the pixel voltage to the data bus with a fixed offset. But this method of operation requires a high data bus impedance. Although methods of shielding are known in the art of electronics, it is very difficult to shield distributed electronic systems, such as large area imagers, against electromagnetic interference. Thus the designer of pixel-gain imagers must choose between non-linearity or interference.